1. Field of the Invention
This invention relates to a lighting apparatus, more particularly to a lighting apparatus capable of reducing variance in intensity of light at different points of an illuminated area.
2. Description of the Related Art
Referring to FIG. 1, a conventional lighting apparatus 1 (e.g., a street light) has a plurality of light emitting units (LEDs) 102 disposed on a flat mounting surface 101 thereof. The light emitting units 102 project light for illuminating a street 2.
A typical government regulation imposed on a street light installation requires two adjacent lighting apparatuses 1 mounted on the street 2 to be 30 meters apart. Each of the lighting apparatuses 1 is capable of projecting two distinctive illumination regions (a center region 202 and an outer region 201). The intensity (E) of the light outputted by the lighting apparatus 1 can be calculated using an equation (E=F/4πR2), where F is the light flux and R is the distance. As can be seen from the equation, the relationship between parameters E and R is inversely proportional. Notably, the farther is the distance from the lighting apparatus (denoted by R), the weaker will be the intensity of the light (denoted by E) illuminated on a surface. Therefore, the light at the center region 202 will be brighter than that at the outer region 201. Further, according to the government regulation, a light contrast ratio between these two regions should not be less than 0.3.
However, the conventional lighting apparatus 1 often provides a light contrast ratio of less than 0.3, which means that there is an unacceptably large variance in intensity of light between the center and outer regions 202, 201. If a driver is driving at a high speed, the presence of the large variance in light intensity at relatively short distances (30 m apart) can pose serious road safety concerns.